The problem with C's undefined behavior is not the undefined behavior itself, but that todays optimizing compilers use UB as a hint to detect "impossible" code

The problem with C's undefined behavior is not the undefined behavior itself, but that todays optimizing compilers use UB as a hint to detect "impossible" code paths which then causes the behavior to really become undefined and unpredictable, also many notionally UB constructions are used in lot of existing code, because before such aggressively optimizing compilers the results were somewhat predictable. Many other languages have concept of UB, sometimes hidden by different wording, even languages that usually target virtual machines (ANSI CL uses the phrase "is an error" for essentially same concept). On the other hand for various VMs the undefined behavior is usually understood as "anything that does not compromise the VM integrity" and similar thing should be true for UB on CPUs (although you should read Intel's errata that contain "causes undefined behavior" as "allows privilege escalation")